1. Field of the Invention
The present invention relates to the use of coherent energy pulses, as from high power pulsed lasers, in the shock processing of solid materials, and, more particularly, to methods for improving properties of solid materials by providing shock waves therein where the laser beam impacts the solid material on a hidden surface. The invention is especially useful for enhancing or creating desired physical properties such as hardness, strength, and fatigue strength.
2. Description of the Related Art
Known methods for shock processing of solid materials, particularly laser shock processing solid materials, typically using coherent energy as from a laser, orient the laser beam normal, i.e., perpendicular to the workpiece.
When particular constraints of processing are created, based on the shape of the material or other geometric factors such as when attempting to laser shock harden integrally bladed rotors (IBR""s), blind bores, slots, or dovetail sections, the laser beam may not have a direct, line of sight access to the area to be shock processed.
Laser shock processing techniques and equipment can be found in the U.S. Pat. No. 5,131,957 to Epstein, along with that of U.S. patent application Ser. No. 08/547,012 entitled LASER PEENING PROCESS AND APPARATUS, assigned to the assignee of the present invention and hereby incorporated by reference.
Known laser shock processing systems tend to form a relatively small, in cross sectional area, laser beam impacting on the surface of the workpiece. This is because a sufficient laser energy must be applied over a particular area to sufficiently work a surface of the workpiece. The smaller the area with the same amount of energy leads to a greater energy per unit area application. The more energy per unit area applied, the deeper the residual compressive stresses are applied to the workpiece.
Laser shock processing of hidden surfaces would benefit particular types and areas of workpieces if such could be accomplished.
One disadvantage in the art of laser shock peening is the failure to teach laser shock processing of hidden surfaces. The current state of the art of laser shock processing contains limitations which prevent effective hidden surface laser shock peening. One limitation which has prevented effective laser shock peening of hidden surfaces is that a pulse of coherent energy may damage a reflective member used to direct a pulse of coherent energy to a hidden surface. The energy density of a pulse of coherent energy necessary to effectively process a workpiece in order to impart deep compressive residual stresses will normally exceed the damage tolerances of a reflective member. This is because the reflective member must necessarily be placed near the target surface due to space constraints near a hidden surface. As a result, prior to this invention, attempting to use a reflective member to redirect a beam of coherent energy to a hidden surface for laser shock peening resulted in damage or destruction of the reflective member due to the converging beam""s decreasing cross-section as it nears the workpiece surface.
A second limitation in the art of laser shock peening which prevents hidden surface laser shock peening is that a non-uniform energy density may be applied to the hidden surface of a workpiece. This is especially an issue when the hidden surface is contoured. For example, the hidden surface could be contoured such that part of the workpiece surface is closer to the last focusing optic of a laser source. The energy density of a beam of coherent energy varies as the distance from the last laser focusing optic increases. This is because the laser beam is converging after the last focusing optic. The shape and design of the last focusing optic affect how the laser beam energy density will vary as a function of distance from the last laser focusing optic. Using current optics in the art of laser shock processing results in the portion of the workpiece contour surface which is closer to the last focusing optic, to be impacted with a pulse of coherent energy having a different energy density than the contour section which is further away from the last laser focusing optic. Consequently, the energy density applied to a hidden surface would be non-uniform.
Another limitation in the art which prevents effective laser shock peening of hidden workpiece surfaces is inconsistently imparted compressive residual stresses over the area processed. For example, when the energy applied to a workpiece surface varies in energy density, the result is non-uniform or inconsistent compressive residual stresses imparted to the workpiece. A consequence of non-uniform compressive residual stresses imparted in a workpiece is the need to reprocess the workpiece with more overlapping laser shock processing spots. As a consequence of either having to reprocess the same spot or providing more overlap of spots, there is an increase in processing time and cost. Moreover, the result of applying a non-uniform energy density across the spot to be processed results in creation of an unpredictable compressive residual stress profile. In addition, non-uniform energy density application to a workpiece results in a less effective and less efficient use of the pulse of energy.
What is needed in the art is a way to modify the laser beam to consistently and uniformly work hidden areas of the workpiece.
The present invention provides a method of laser shock peening that can be used in a production environment to apply laser shock processing treatment to hidden surfaces once thought not applicable for treatment.
The present invention includes use of a reflective member inserted into a recess of the workpiece. The reflective member is created to reflect an inbound laser beam to the hidden surface within the workpiece. Different geometries and forms of the reflective member are given, some dependent on the shape of the recess.
The term recess as used in this application is that of an opening, port, hole, channel, or other space within the workpiece. The term hidden surface as used in this application is an interior surface of the workpiece, not normally available for direct line-of-sight laser processing. Typical recesses with hidden surfaces include, the interior surfaces that define holes and blind bores, the interior roof of dovetail slots as can be found in aircraft gas turbine disks, and other similar openings and ports in workpieces.
The invention, in one form thereof, is a laser peening method for processing a hidden surface of a workpiece. The hidden surface is disposed within a recess having an opening. The laser peening method comprises the steps of inserting a reflective member into the recess. Means are provided for preventing damage to the reflective member. A pulse of coherent energy is directed to reflect off of the reflective member and impact the hidden surface of the workpiece to create a shock wave.
In one embodiment, the reflective member is a focusing mirror. In an alternate embodiment, the means for preventing damage to the reflective member comprises providing a gap between a transparent overlay applied to the workpiece and the reflective member. In an alternate embodiment, means for preventing damage to the reflective member comprises providing the pulse of coherent energy at a minimum energy density when reaching the reflective member and an operational energy density when the pulse impacts the workpiece.
In one specific further embodiment, the reflective member is a flow of fluid flowing along the length of a recess having an opening, such as a dovetail shape. The laser processing beam would be incident on the flow of fluid and would be reflected to the workpiece hidden surface by refraction and total internal reflection in the fluid flow.
The invention, in another form thereof, is a laser peening method for processing a hidden surface of a workpiece. The hidden surface is disposed within a recess having an opening. The laser peening method comprises the steps of inserting a reflective member having a geometry into the recess. The reflective member geometry is determined such that when a pulse of coherent energy reflects off of the reflective member, the pulse will impact the workpiece surface with a substantially uniform energy density being applied to the workpiece surface. A pulse of coherent energy is directed to reflect off of the reflective member and impact the hidden surface of the workpiece to create a shock wave therein. In one particular embodiment, the hidden surface has a contour. In a further, alternate embodiment, the recess is dovetail shaped.
The invention, in yet another form thereof, is a laser shock peening method for processing a workpiece. A reflective member is formed from a fluid. A pulse of coherent energy is directed to reflect off of the reflective member and impact the workpiece to create compressive residual stresses therein.
The invention, in another form thereof, is a laser peening apparatus for improving properties of a workpiece by providing shock waves therein. An energy absorbing overlay is applied to the workpiece. A laser generates a laser beam which is operatively associated with the energy absorbing overlay to create a shock wave on the workpiece. A reflective member is composed of fluid and is operatively associated with the laser generator. In a further embodiment, a high speed nozzle forms the reflective member.
An advantage of the present invention is that hidden surfaces of a workpiece may now be effectively laser shock processed.
Another advantage of the present invention is the application of a pulse of coherent energy to a workpiece having a contoured surface in which the energy applied to the workpiece surface is substantially uniform throughout the processed spot. The advantage of uniformly applying energy across a process spot provides for consistent imparting of compressive residual stresses to a workpiece. In addition, more predictable compressive residual stresses may be applied to a workpiece. In turn, fatigue life, hardness, and strength may be enhanced as compared with the application of a non-uniformly applied energy density to the workpiece.
Another advantage is that the hidden surfaces may be laser shock processed in a production environment.
A further advantage of the present invention is the ability to precisely control the laser beam inside the workpiece.
Yet another advantage of the present invention is that by applying lower powered laser energy through a majority of the opening of the recess, the energy per unit area remains small, reducing negative effects, until reflected and focused by the reflective member. Such lower power-density laser beam use, spread out over the recess opening, but later focused to the power density necessary for laser shock processing, increases the effectiveness, and possibly the operational lifetime, of the reflective member or reflective surface.
Yet another advantage of the present invention is the prevention of plasma formation on the reflective member, for example, a focusing mirror.
A further advantage of the present invention is that, in one embodiment, the reflective member is continuously renewed. When the reflective member is composed of a fluid, the reflective member will not suffer the damage limitations of conventional reflective members. Consequently, there is no need to replace or repair the reflective member between successive laser shock peening cycles. Therefore, the speed by which one can laser shock peen a workpiece is not limited by having to replace the reflective member.
An additional advantage of using a fluid reflective member is the ability to modify the reflective optic properties of the reflective member. By varying the flow of fluid and the fluid shape, the shape of the reflective member is altered. Altering the shape of a reflective member will consequently change the reflective characteristics of the reflective member. By changing the geometry of the reflective member, one is able to modify the energy density of a beam of coherent energy applied to a workpiece and the location on the workpiece to be processed.